Mainly:
- access to their AMI library: being a popular platform, they may have AMIs for which Proxmox currently has no counterpart
- portability: such that I can use a PVE environ for building/testing, then deploy to EC for actual running; or, suppliment my PVE with EC systems; or, temporarily shift functionality because of anticipated high use.
It'd be cool (in an "enterprise" way) to be able to build a corporate infrastructure that is a "private cloud" that can play nice in a variety of ways with larger clouds like Amazon's EC.